This invention relates to noise filters for electrical measurements of process variables, and primarily to filters for filtering the noise from signals to controllers so as to prevent controller response to noise in signals. The prior art has utilized standard filter techniques to filter out of signals to controllers all high frequency components which are uncontrollable. In addition, the prior art has also utilized the arrangements of U.S. Pat. Nos. 3,419,772 and 3,633,009 in minimizing the response of controllers to noise signals. Those arrangements use a joint probability function of signal amplitude and duration with the joint probability function being used to modify the gain of the control system, for example.
The present invention provides a means for filtering noise signals from a controller input with apparatus which is considerably simplified with respect to that required in the joint probability approach and which is more selective than the standard filter techniques. There is, therefore, provided by the invention an improved controller input filter which provides improved filtering action at a minimum cost.